(1) Field of the Invention
The present invention relates to a system and method for automatically controlling a vehicle speed to a desired cruise speed applicable to a vehicle in which an automatic transmission is mounted and using a lock up control and gear range change control of the automatic transmission.
(2) Background of the art
Various types of automatic vehicle speed controlling systems have been installed in automotive vehicles for controlling the vehicle speed so as to reach a set cruise speed and maintain the vehicle speed at the set cruise speed without operation of accelerator pedal.
Such automatic vehicle speed controlling systems are called ASCD's (Automatic Speed Control Devices).
A Japanese Publication NISSAN SERVICE WEEKLY INFORMATION (SHUHOU) No. 491 "Introduction to Bluebird; U 11 type" II-141 page (published on October, 1983) discloses an ASCD mounted in an automatic transmission (A/T) mounted in the vehicle in which, when the vehicle runs in an over drive range OD (i.e., in the highest gear range), and vehicle speed is reduced due to increase in the engine load and deviates largely from a set cruise speed, an ASCD control unit produces an OD inhibit signal to an A/T control unit so that the A/T gear range is shifted down to recover the set vehicle cruise speed. ( A U.S. Pat. No. 4,697,478 issued on Oct. 6, 1987 also exemplifies an automatic vehicle speed control system applied to an automatic transmission mounted vehicle.)
However, since the A/T gear range is set to downshift to a third gear from the OD range at the OD inhibit signal in order to recover the set vehicle cruise speed in the ASCD mode, large stepwise changes in engine output torque and revolutional speed occur, as shown in FIG. 9. Such changes cause a sudden jolt to the vehicle body so that vehicle comfort is degraded.